The present invention relates to radio frequency (RF) power controllers. More particularly, the present invention relates to RF power controllers with offset voltage cancellation.
An RF power amplifier may output power to, for example, an antenna that acts as a load drawing power from the RF amplifier. An RF power controller is a circuit that regulates the output power of the RF power amplifier. A power control input signal tells the RF power controller what power level the RF power amplifier should send to the antenna when power is required, which may be referred to as "ENABLE" mode. Thus, the ENABLE mode is the period of time during which the antenna draws power from the RF power amplifier. The RF power controller provides an output voltage signal to the RF power amplifier in response to the power control input signal. The output voltage indicates to the RF power amplifier the power level that the RF power amplifier needs to output. The relationship between the output voltage of the RF power controller and the power output by the RF power amplifier in response to the output voltage forms a transfer curve for the RF power amplifier.
The RF power controller regulates the output power signal of the RF power amplifier to the level indicated by the power control input signal using a feedback signal The feedback signal from the RF power amplifier indicates to the RF power controller how much power the RF power amplifier is outputting at a given point in time. The output power requirement of the RF power amplifier may vary as the distance to the base station changes. The RF power controller uses the feedback signal to determine whether the RF power amplifier is outputting the power level called for by the power control input signal. In response to the feedback signal, the RF power controller varies the output voltage signal to the RF power amplifier. The feedback signal closes a loop called the power control loop around the RF power amplifier and the RF power controller.
The power control input signal also indicates when the RF power amplifier no longer requires output power. The RF power controller responds by reducing its output voltage to zero which directs the RF power amplifier to discontinue its power output. The RF power amplifier is essentially dormant during standby mode which follows each enable mode. The duty cycle of the RF power amplifier includes the time the RF amplifier is in ENABLE mode and in STANDBY mode.
RF power controllers can be used in many applications using RF power amplifiers, such as cellular telephones, and wireless data modems. It is important in these applications that the RF power controller regulate the RF power amplifier output signal as accurately as possible. Offset voltages that are associated with the power control input signal and with circuitry within the RF power controller itself can adversely effect the accuracy of the RF power controller output voltage signal.
The problem of offset voltages having an adverse affect on the output signal of RF power controllers has become increasingly important as the power requirements for RF power amplifiers in many applications have decreased, thereby increasing the impact of those offsets. In some cellular phone applications, the output power requirement can be very low (e.g., 5 dBm). In instances when output power requirements are low, offset voltages that cause inaccuracies in the output of the RF power controller are even more important because errors in the RF power controller output may affect a large percentage of its output voltage. Errors in the RF power controller's output voltage often result in inaccurate power output signals from the RF power amplifier. Offset voltages often are difficult to eliminate from the power control loop because they can vary in response to changes in output power levels, power supply inputs, and operating temperature.
Negative offset voltages can cause unwanted steps in the output voltage of the RF power controller. For example, negative offset voltages in the power control loop can cause delays in the start-up of the power control amplifier at the beginning of the ENABLE cycle. The delays may be associated with the turning ON of transistors in the power control amplifier. Once the delay is over, the output voltage of the power control amplifier steps up rapidly as the power control amplifier begins to output voltage in order to acquire the poewr control input signal. Positive offsets can also cause spurious steps in the output voltage of the power control amplifier. Moreover, positive offsets can cause the output voltage of the power control amplifier to step up before voltage is applied to the power control input at the beginning of the ENABLE cycle.
The rapid increase in voltage associated with steps up in output voltage also can cause spurious side band frequencies in the output power of the RF power amplifier that can, themselves, cause interference on adjacent channels. For example, spurious side band frequencies can cause noise in adjacent channels used by other cellular phones. Offset voltages also can prevent an RF power controller from ramping up its output voltage at steady rate, thereby causing spurious side band frequencies. For these reasons it is advantageous to eliminate offset voltages associated with RF power controllers.
Prior art circuits have employed various methods to cancel offset voltages that can creep into the power control loop of RF power controllers. An RF power controller may contain a power control amplifier that amplifies the power control input signal to produce the controller's output voltage. Voltage offsets associated with an RF power controller can be reduced by connecting the input of the power control amplifier to a trim potentiometer, to cancel out the offset voltages. However, manual trimming using a potentiometer is expensive and requires additional PC board area. Also, temperature changes and supply voltage changes may not be accounted for.
Another prior art technique for canceling offsets involves adding negative offsets to the power control amplifier in order to cancel positive offsets associated with other circuitry in the power control loop. However, it may be difficult to match the negative offset with the positive offset to provide complete cancellation. This may especially be a problem when the offset voltages change with operating temperature and power supply input. If too much negative offset is added to the control amplifier, a net negative offset is present that can cause steps in the output voltage at the start of the enable mode.
Another known technique for canceling offsets involves a lookup table that takes into account offset voltages for different RF power levels, input supply voltages, and temperatures. Offset voltages associated with particular RF power controllers may vary depending upon, e.g., RF output power levels, input supply voltages, and operating temperature. RF power controller circuit boards can be tested to determine the offset voltages that are associated with the RF power controller at different RF power levels and temperatures for each individual board. These offset voltages can then be saved in a lookup table. During the operation of the RF power controller, an offset voltage value for a particular operating temperature, input supply voltage, and RF power level can be accessed from the lookup table by additional circuitry to determine the voltage needed at the input of the power control amplifier to cancel out the offset voltages.
Under these circumstances, however, lookup tables must be generated for each individual PC board to fully take into account variations over temperature, input supply voltage, output power levels, and the initial offset voltages. These tables may require complicated algorithms. In addition, the lookup table technique may result in inaccuracies in canceling the offset voltages associated with the RF power controller because the temperature coefficients of offset voltages are unpredictable. An offset voltage associated with particular temperatures may change over time and cannot be accurately predicted with a lookup table. Therefore, prior art circuits using lookup tables to cancel out offset voltages often also use trim potentiometers to cancel out additional offsets that cannot be predicted by a lookup table.
In view of the foregoing, it would be desirable to provide techniques for canceling offset voltages associated with RF power controllers accurately over varying output power requirements, supply voltages, and operating temperature ranges.
It would further be desirable to provide techniques for canceling offset voltages associated with RF power controllers using a minimal amount of additional circuitry.
It would be additionally desirable to provide techniques for automatically canceling offset voltages associated with RF power controllers during each duty cycle that does not need to be manually adjusted or modified for each individual RF power controller.